I will differ based on my expierience on my old Farmall, and friends expierience who have pulling tractors. The clutches can be slipped, but no where near like a car. The tractor is made to pull hard. If you slip a clutch on something made to pull big bottom plows or discs, there would be too much slippage and clutch damage.
If you pull a wagon or float with people on it, you have to be careful. I made this post, because I rolled my 7 year old off the back of our float last year. I slipped the clutch as best I could, at low rpm, in first gear. Even though that Farmall will only do a mile per hour in first gear, it lurched forward. Ian was sitting by the rear of the trailer, and took a tumble off. Fortunately, this was during a test run before the parade. After that, I made a grab/cross bar for the back of the float; I didn't want the chance of someone falling off again. The parade was successful, with the Cub Scout pack winning 1st place in the parade.
Letting the foot slip off the clutch would be obvious, albeit embarassing. It is not however, the exception.
Not all tractors are the same. Not all clutches react the same, whether worn or new. Some may slip easier than others...
The main point is that the original poster is aware of the possibility, and tries to be safer with the knowledge. It is good to try these things out before pulling with people on the trailer, and be safe in doing it. It is one thing to pull a box scraper or a plow. But, when you got people on the wagon behind, it's a bit different driving.